1. Field of the Invention
The present invention relates to optical beam alignment systems, and more specifically, it relates to compact, non-invasive optical beam alignment systems for centering and pointing of an optical beam from a remote location.
2. Description of Related Art
Devices to align an optical beam in space are well known in the art. The prior art in the area of optical beam alignment typically requires two optical sensors, or apertures with a detector, with each sensor located at a different point in space. (An example of a sensor can be a pinhole aperture mounted onto an optical detector.) The dual-sensor requirement sterns from the fact that an optical beam can be described in geometrical terms as a straight line. The precision of an alignment tool improves as the physical separation between the pair of sensors increases. The trajectory of an optical beam in space can be specified by a given point in space (e.g., its centering location at one aperture) and by the slope of the beam (e.g., the pointing direction of a beam from a given aperture to a second aperture).
However, situations exist where the separation between the pair of sensors is limited to a small distance. Moreover, there exist cases where is it highly impractical, and, in other cases, deleterious, to locate one or a pair of optical detectors at different locations within a confined system with a separation distance sufficiently great to determine the spatial location of an optical beam with a specified precision. Examples of adverse environments include high vacuum chambers, high temperature or cryogenic environments, high radiation situations, etc.
The prior art also includes pointing and tracking techniques, which can he open loop or servo-controlled to determine and position a laser beam along a given propagation path. Such systems also require the determination of two parameters, namely, the centering of a laser beam at a given point in space, and its pointing direction relative to that point. Again, a pair of sensors is required for this operation. This is typically accomplished using a pair of pinhole apertures, positioned at locations such that an optical beam passes through both pinholes. A variation on this approach is to place optical beam splitters at the approximate locations through which the beam must pass. Each beam splitter reflects a small fraction of the optical beam power to a respective precision pinhole/detector module. The location of the beam as it passes through the pair of pinhole detectors will thusly define its spatial coordinates. The beam splitters typically allow for most of the power in the optical beam to pass through the beam splitters. Therefore, the location of the beam is established in space with minimal loss of optical power required for measurement and alignment purposes. The prior art, however, requires the presence of a pair of spatial fiducial locations, each with its respective detector, and with a minimal separation between them to determine the propagation direction of the beam with a given precision.
The need for a pair of such sensors adds complexity to the system, since two pinholes, two detectors, and, possibly, two beam splitters are required, in addition to optical mounts, fixtures and electronic processors. In addition, the pair of beam splitters must be of high optical quality as to not distort the main beam as it is directed to its target. Also, the beam splitters must be sufficiently large in cross section as to not obscure the main beam. In addition, although the beam splitter will preserve the angle of incidence of the beam as it exits the splitter, it will, however, physically displace the transmitted beam a small distance parallel to the incident beam, the displacement depending on the thickness of the beam splitter. These requirements add complexity, cost and weight as well as requiring space for their installation, and, access for their maintenance.
The prior art also includes various forms of aiming devices including telescopes, gun sights, and surveyor apparatus. In these systems, to align the beam, a viewer (or camera) has to focus first on one mask at given location within the sight (such as a crosshair, located at the entrance to the instrument), and, then focus on another sight, typically located at the extreme opposite end of the device. The presence of a pair of separated crosshairs does not permit simultaneous measurements, given the necessity to refocus ones eye or machine vision system, resulting in a greater time to complete the measurement. As an example, one defocused sight may interfere with the viewer or camera during the time that one attempts to concentrate and focus on the other sight. Also, since the patterns are fixed in time, the sight geometry may not be optimal during the convergence process. Hence, there is a need to decouple the pair of sights as well as to provide a means that can enable one or both sight-patterns to dynamically change in real-time, as necessary, so that the path to alignment convergence is minimized in terms of residual error, time, and, moreover, accommodating to random effects such as vibrations, obscuring objects in the field of view, and beam wander.
Therefore, there is a need to relax the critical cost, weight and complexity requirements to provide a pair of spatially dispersed sensors necessary to ascertain, set and maintain the centering and pointing parameters of an optical beam, even in the presence of platform vibration, target obscuration and beam wander. Moreover, in some cases, it is highly desirable to enable noninvasive placement of a compact, rugged diagnostic completely external to a given structure (with optical access).